Mammalian respiration occurs by gas exchange through air sacs or alveoli in the lungs, and thus is referred to as "alveolar ventilation." FIG. 1 (prior art) shows pulmonary passageways which deliver and remove respiratory gases to and from the alveoli of lungs 200. In successive order, these passageways include the larynx 202, trachea 204, bronchus 206 and segmental bronchi or bronchioles 208. The bronchioles 208 terminate in small clusters of grapelike air sacs 210 (the alveoli) where the gas exchange occurs.
FIG. 2A (prior art) shows one alveolus 212 of the alveoli 210 and FIG. 2B (prior art) diagrammatically represents the gas exchange through the alveolus 212. A network of blood capillaries 214 covers or surrounds the alveolar walls 216. The gas-filled interior region of the alveolus 212 and the network of capillaries 214 are separated by less than 0.5 .mu.m of intervening tissue. The gas exchange in alveolar lungs can be modeled as a ventilated pool 218, as shown in FIG. 2B.
During liquid ventilation, the pulmonary passageways of the lungs are filled with a breathable liquid which has the ability to deliver oxygen to, and remove carbon dioxide from, the pulmonary system. Two common types of liquid ventilation processes include "total liquid ventilation" and "partial liquid ventilation."
In a total liquid ventilation system, a breathable liquid is oxygenated and pumped or instilled into the lungs during an inspiratory breathing stage. When the breathable liquid reaches the alveoli, the oxygen in the breathable liquid diffuses into the blood of the capillaries surrounding individual alveolus. Correspondingly, carbon dioxide in the blood diffuses into the breathable liquid. The breathable liquid is then pumped out or removed from the lungs during an expiratory breathing stage. The expired liquid is scrubbed to remove the carbon dioxide, reoxygenated and returned to the lungs during a subsequent inspiratory breathing stage. A respirator typically performs the breathing stages. Such systems are described in U.S. Pat. Nos. 5,335,650 and 5,158,536, both of which are incorporated by reference herein in their entirety.
In a partial liquid ventilation system, a breathable liquid is instilled into the lungs and remains therein. This system is often employed when the lungs are collapsed since the volume of the breathable liquid functions to expand the lungs. The breathable liquid fills the alveoli. Then, respiratory gas is pumped into and out of the lungs. Oxygen carrying inspiratory respiratory gas interacts with the breathable liquid and releases the oxygen to the breathable liquid. In turn, the breathable liquid releases the oxygen into the blood surrounding the alveoli in the same manner as described above in the total liquid ventilation system. Likewise, carbon dioxide in the blood diffuses into the breathable liquid, which in turn, diffuses into areas of the lungs not taken up by the breathable liquid. During the expiratory phase, expiratory gas (including the carbon dioxide) exits the lung. As noted above, during partial liquid ventilation, the breathable liquid remains in the lungs, acting as an exchange medium for the oxygen and carbon dioxide entering and exiting the lungs. Partial liquid ventilation, as performed today, is not a closed loop system.
Breathable liquids employed today have various vapor pressures. During partial liquid ventilation, a small amount of the breathable liquid will volatilize or vaporize with each breathing cycle by saturating the respiratory gas. That is, the vapor pressure of the breathable liquid causes gas vapors coming off the liquid to saturate the respiratory gas as the gas flows through and around the liquid. During the expiratory phase, the saturated or partially saturated gas leaves the respiratory system. Since partial liquid ventilation is not a closed loop system, the volatilized breathable liquid must eventually be replaced by a new instillation of breathable liquid into the patient's lungs.
During partial liquid ventilation, a portion of the breathable liquid is also lost due to evaporation into the lungs. Some of this evaporated liquid becomes absorbed by the lungs and eventually leaves the patient's body by transpiration through the skin. Significant problems still exist in performing total and partial liquid ventilation. During total liquid ventilation, the breathable liquid also undergoes volatilization and dissolves in the expiratory liquid. Total liquid ventilation systems employed today scrub dissolved carbon dioxide from the expiratory liquid before the gas is reoxygenated and cycled back into the patient's lungs. This process occurs in an oxygenator/diffuser circuit. Not all of the carbon dioxide is scrubbed from the diffuser. Furthermore, none of the vaporized breathable liquid is recovered in the scrubber. Instead, it is vented to the environment. Accordingly, the system must periodically add more breathable liquid from a storage reservoir. This increases the cost of the liquid ventilation process since breathable liquid is expensive (e.g., as much as $2.00/ml).
During partial liquid ventilation, an operator must continually monitor the process to ensure that sufficient alveolar ventilation is occurring. One important aspect of the monitoring is to ensure that there is a sufficient quantity of breathable liquid in the lungs to promote the desired amount of alveolar ventilation. Alveolar ventilation can be compromised if the volume of liquid in the lung becomes too small.
Current techniques for measuring the amount of breathable liquid in the lungs are inaccurate and inadequate. One technique employed today involves merely replenishing the supply of breathable liquid in the lungs until they are filled. This is supposedly accomplished by visualizing a meniscus of PFC in the endotracheal tube. However, it is not always necessary or desirable to completely fill the lungs to achieve the desired amount of alveolar ventilation. Accordingly, the operator does not know for sure how much breathable liquid to add as volatilization depletes the store of liquid.
Oftentimes, the breathable liquid becomes maldistributed throughout the lungs due to patient movement or density differences which cause liquids to settle and gases to rise. For example, some bronchioles may have little or no breathable liquid to supply the alveoli at their distal ends, whereas other bronchioles may be overfilled. This maldistribution may also cause insufficient interaction between the breathable liquid and the respiratory gas. Atelectasis may also cause insufficient interaction between the breathable liquid and the respiratory gas. Atelectasis is the collapse of the expanded lung or the defective expansion of the pulmonary alveoli at birth. Currently, the operator of a liquid ventilation system has no sure technique for gauging whether insufficient alveolar ventilation is the result of an inadequate quantity of breathable liquid in the lungs, maldistribution of the breathable liquid or atelectasis.
Furthermore, the volatilized liquid in the expiratory gas is vented to the environment in the same manner as the total liquid ventilation system. Again, this loss of a valuable substance raises the cost of the overall process.
The inability to accurately detect the amount of breathable liquid in the patient's lungs complicates effective patient management.
Accordingly, there is still a need for apparatus and methods to improve liquid ventilation processes. Specifically, there is a need for apparatus and methods which allow the operator to more accurately gauge the amount and distribution of breathable liquid in a patient undergoing partial liquid ventilation, the amount being lost due to vaporization or through other evaporative channels and the amount of interaction between the breathable liquid and respiratory gases. There is also a need for apparatus and methods to scavenge or recover vaporized breathable liquid from expiratory gas and to monitor the efficiency of the recovery equipment. The current invention fills these needs.